In recent years, many rubber products such as belts, tires, rolls and mold goods come to be used together with an oil heated under high temperature and high pressure for a long time, and deterioration of rubber products under such conditions always brings significant problems. If deterioration of rubber products is remarkable, very much time and labor are needed for maintenance or replacement of such deteriorated rubber products. Sometimes deterioration of rubber products may cause a large accident.
Conventional polymers that have excellent oil resistance and can be durable to continuous use under such high temperature (about 120-150.degree. C.) environment are acrylonitrile-butadiene copolymer rubber (NBR), acrylic rubber (ACM), ethylene-acrylate copolymer rubber (AEM), ethylene-acrylate-vinyl acetate copolymer rubber (ER), ethylene-vinyl acetate copolymer rubber (EVM), chlorosulfonated polyethylene rubber (CSM), chlorinated polyethylene rubber (CM), and hydrogenated acrylonitrile-butadiene copolymer rubber (HNBR) obtained by hydrogenating a conjugated diene unit of acrylonitrile-butadiene copolymer rubber (NBR).
On the other hand, in rubber compositions, it is known that an organic peroxide-crosslinked rubber composition using organic peroxide has excellent heat resistance as compared with a sulfur-vulcanized rubber composition using sulfur in vulcanization.
However, the organic peroxide-crosslinked rubber composition does not generally contain sulfur that reacts with a metal for adhesion. As a result, such an organic peroxide-crosslinked rubber composition has poor adhesiveness to a metal surface and does not adhere to a brass that is plated on a metal surface. Therefore, rubber products composed of a combination of such an organic peroxide-crosslinked rubber composition and a metal such as a brass-plated steel plate have defects in that separation occurs at the interface between a rubber layer and a plated layer, resulting in breakage of the products.
For example, hydrogenated acrylonitrile-butadiene copolymer rubber having high rate of hydrogenation have the characteristics of excellent heat resistance and high modulus, and due to such characteristics, the copolymer rubbers are used for a wide variety of applications. However, since the copolymer rubbers have a low iodine number, it is necessary for the copolymer rubbers to be crosslinked with organic peroxide in using the copolymer rubbers. Thus, sulfur can not be used as a crosslinking agent. Therefore, it is extremely difficult to directly adhere hydrogenated acrylonitrile-butadiene copolymer rubber having high rate of hydrogenation to a metal. For this reason, composite products of rubber and metal, such as belts, tires, rolls and molded goods, could not conventionally been produced.
To overcome the above disadvantages, regarding organic peroxide-crosslinked rubber composition of hydrogenated acrylonitrile-butadiene copolymer rubber, a method is proposed which improves adhesiveness with brass by adding 2,4-dimercapto-6-R-1,3,5-triazine as described in, for example, Japanese Patent Application Laid-open No. Sho 62(1987)-104864. However, this method had the disadvantage that although adhesiveness is improved, modulus and heat resistance decrease due to the reaction between organic peroxide crosslinking agent and 2,4-dimercapto-6-R-1,3,5-triazine. Further, Japanese Patent Application Laid-open No. Hei 3(1991)-31338 proposes a method of improving adhesiveness to brass and modulus by adding 2,4-dimercapto-6-R-1,3,5-triazine and triallyl isocyanurate. However, this method still had the disadvantage that heat resistance is poor.
On the other hand, Japanese Patent Application Laid-open No. sho 55(1980)-125155, for example, discloses that a polymer composition comprising an organic peroxide-crosslinkable polymer, an organic peroxide, an epoxy resin and 2,4-dimercapto-6-R-1,3,5-triazine has good adhesiveness to brass. However, from the results of our investigation and the contents of the working examples, Japanese Patent Application Laid-open No. Sho 55(1980)-125155 utilizes a reaction between chlorine in a chlorine-containing polymer such as chlorinated polyethylene and 2,4-dimercapto-6-R-1,3,5-triazine, reaction between 2,4-dimercapto-6-R-1,3,5-triazine and an epoxy resin and a reaction between 2,4-dimercapto-6-R-1,3,5-triazine and copper in brass. Hydrogenated acrylonitrile-butadiene copolymer rubber that is a chlorine-free polymer does not cause a reaction for adhesion, with the result that adhesiveness is insufficient. Thus, Japanese Patent Application Laid-open No. Sho 55(1980)-125155 had such disadvantage that adhesiveness is not yet sufficient, considering the technical level presently required.
Accordingly, it is extremely difficult to directly adhere hydrogenated acrylonitrile-butadiene copolymer rubber having high rate of hydrogenation to a metal such as brass, and also to obtain a rubber composition having high modulus and high heat resistance. Thus, there is remarkable restriction in the production of composite products of a rubber and a metal, such as belts, tires, rolls and molded goods.